In fluid process control applications in chemical, pulp and food processing plants, different types of pressure transmitters are used. These types include the absolute pressure transmitter that measures a process pressure relative to a vacuum, the gage pressure transmitter that measures a process pressure relative to local atmospheric pressure, and the differential pressure transmitter that measures a difference between two process pressures. Pressure transmitters also typically measure pressure over only a limited range of pressure within specified accuracy, and typically a transmitter will be manufactured in 5 or more overlapping ranges, each specified to measure pressure accurately over about a 100:1 (xe2x80x9cturndownxe2x80x9d) range to fill application needs up to 69,000 kPa (about 10,000 psi).
The different types and ranges are generally not fully interchangeable. A large fluid process control plant will typically have dozens or hundreds of pressure transmitters of all three types and differing ranges, leading to costly problems with stocking many types of replacement transmitters, and potential for damage due to overpressure when a low range pressure transmitter is installed in error in a high pressure installation.
A transmitter can be used interchangeably in absolute, gage, and differential pressure measurement applications and has adequate range so that fewer types of transmitters can be stocked to fill the needs of a process control plant. The pressure transmitter generates differential and non-differential outputs.
The transmitter comprises two absolute pressure sensors adapted to sense pressures P1 and P2 at process inlets, and a third absolute pressure adapted to sense atmospheric pressure. A transmitter circuit couples to the three absolute pressure sensors, and the transmitter circuit generates differential and non-differential type outputs, such that the transmitter is interchangeably adaptable between differential and non-differential installations.
The transmitter has three absolute pressure sensors, and the three pressures P1, P2 and P3 are sensed independently. The conventional arrangement where a single differential sensor measures the differential pressure (P2xe2x88x92P1) is avoided, and thus there is no need to sense line or static pressure to provide line pressure compensation for the differential pressure measurement. The complexity of the linearization and compensation task for each sensor is reduced because each sensor is only subjected to one pressure. Moreover, multiple types of outputs can be provided by a single pressure transmitter, improving interchangeability of replacement transmitters and reducing the cost of stocking and manufacturing large numbers of transmitter types.
The availability of multiple outputs makes it possible for one transmitter to perform up to three pressure measurements. Each of the process inlets can be connected to separate process pressures and the transmitter will provide separate absolute pressure outputs representing the process pressures. The atmospheric inlet, which is threaded, can be connected to a third pressure rather than being vented to the atmosphere, allowing a third measurement to be taken. The availability of the multiple outputs also makes it convenient and economical to perform redundant measurements on a single process pressure for added reliability in case of a sensor failure. With three sensors available in the transmitter, majority voting logic can be included in the transmitter to allow redundant measurement to continue with two sensors after one sensor has failed.